More than 500 million users have upgraded to Google+ accounts, while 235 million are actively using Google+ features (like +1’ing apps, or following friends in Google Search), Google Senior Vice President Vic Gundotra said in a blog post today.

135 million users are active in the Google+ stream proper, the service’s equivalent of Facebook’s news feed. The numbers show significant growth from June, when Google+ only had 250 million total accounts and 150 million monthly active users.

“This enthusiasm, we think, stems from our building tools that build real relationships — in a live hangout, around a breathtaking photo, or with an inner circle of friends,” Gundotra wrote.

While far from Facebook’s 1 billion monthly active users, the growth shows that Google+ isn’t something to be ignored. Even if you don’t use the service as a true Facebook replacement, it’s becoming increasingly complementary to Google’s other services. That makes it something of a social networking Trojan Horse for Google, and an increasingly bigger threat to Facebook.